squeel_test.rb

subquery = Article.where(:person_id => 1).select(:id).order{id.desc}.limit(1)
Person.where(:id => 1).select{[id, name, subquery.as('last_article_id')]}.to_sql
=> SELECT "people"."id", "people"."name", 
     (
       SELECT  "articles"."id" FROM "articles"  
       WHERE "articles"."person_id" = 1 
       ORDER BY "articles"."id" DESC LIMIT 1
     ) last_article_id
   FROM "people"  WHERE "people"."id" = 1